1. Field of the Invention
The invention relates to a torque detector including a magnetic flux collecting unit that collects magnetic fluxes from a magnetic yoke, and a sensor housing formed integrally with the magnetic flux collecting unit, and relates to a steering system including the torque detector.
2. Description of Related Art
A conventional torque detector includes a magnetic flux collecting unit including magnetic flux collecting rings, a ring holder and a magnetic shield, and a sensor housing. The torque detector is configured such that the magnetic flux collecting unit is fixed to the sensor housing with the magnetic flux collecting unit inserted in an insertion hole formed in the sensor housing. An example of such a conventional torque detector is described in Japanese Patent Application Publication No. 2008-249598.
In the conventional torque detector, there is a possibility that water may enter the torque detector through a gap between the magnetic flux collecting unit and the sensor housing. Therefore, in order to enhance the water-tightness of a torque detector, there has been proposed a torque detector 200 configured such that a sensor housing 220 is formed integrally with a magnetic flux collecting unit 210 by supplying resin onto the outer periphery of the magnetic flux collecting unit 210 as illustrated in FIG. 12.
The magnetic flux collecting unit 210 includes a magnetic flux collecting holder 211, a magnetic flux collecting ring 212 and a magnetic shield 213. In the magnetic flux collecting unit 210, the magnetic flux collecting ring 212 is fitted to an inner peripheral face 211X of the magnetic flux collecting holder 211, and the magnetic shield 213 is fitted to an outer peripheral face 211Y of the magnetic flux collecting holder 211.
The magnetic shield 213 is formed of a metal plate. The magnetic shield 213 has a C-shape in a planar view. The magnetic shield 213 is in contact, at its inner peripheral face 213X, with the outer peripheral face 211Y of the magnetic flux collecting holder 211. The magnetic shield 213 has a shield body 213A and shield end portions 213B. The magnetic shield 213 is configured such that the shield end portions 213B are continuous with respective end portions of the shield body 213A in the circumferential direction. In the magnetic shield 213, an angle formed between the shield body 213A and the radial direction of the sensor housing 220 (hereinafter, referred to as “first angle AR1”) and an angle formed between each shield end portion 213B and the radial direction of the sensor housing 220 (hereinafter, referred to as “second angle AR2”) are equal to each other. In the magnetic shield 213, the outer peripheral face of the shield body 213A and the outer peripheral face of each shield end portion 213B are orthogonal to the radial direction ZB of the sensor housing 220, and therefore, both the first angle AR1 and the second angle AR2 are 90°.
The sensor housing 220 is in close contact with an outer peripheral face 213Y of the magnetic shield 213 and corner portions 213D of the shield end portions 213B. The corner portions 213D of the shield end portions 213B are formed by end faces 213C of the shield end portions 213B and the outer peripheral face 213Y of the magnetic shield 213.
In the torque detector 200, the linear expansion coefficient of the sensor housing 220 is greater than the linear expansion coefficient of the magnetic shield 213. Therefore, when the temperature of the torque detector 200 changes, the amount of thermal expansion of the sensor housing 220 is greater than that of the magnetic shield 213, and the amount of thermal contraction of the sensor housing 220 is greater than that of the magnetic shield 213.
Accordingly, when the torque detector 200 is cooled, the sensor housing 220 presses the shield end portions 213B due to the difference between the thermal contraction amount of the sensor housing 220 and the thermal contraction amount of the magnetic shield 213. Specifically, the sensor housing 220 presses the outer peripheral face 213Y of the magnetic shield 213 radially inward, due to the difference between the amount of thermal contraction of the sensor housing 220 in the radial direction and the amount of thermal contraction of the magnetic shield 213 in the radial direction. Further, the sensor housing 220 presses the end faces 213C of the shield end portions 213B in the circumferential direction, due to the difference between the amount of thermal contraction of the sensor housing 220 in the circumferential direction and the amount of thermal contraction of the magnetic shield 213 in the circumferential direction.
As illustrated in FIG. 13, as the result of analysis conducted by the applicant according to the finite element method (FEM), the applicant found that a reaction force (hereinafter, referred to as “pressing reaction force”) against the force with which the sensor housing 220 (refer to FIG. 2) presses the outer peripheral faces of the corner portions 213D of the shield end portions 213B radially inward, is greatest when the torque detector 200 is cooled. Thus, the sensor housing 220 is pressed radially outward by the outer peripheral faces of the corner portions 213D of the shield end portions 213B due to the pressing reaction force. As a result, large thermal stresses are generated in portions of the sensor housing 220, which surround the corner portions 213D of the shield end portions 213B from the outside in the radial direction.